The invention relates to a linear position measuring system and to a method for determining the position of a carriage in relation to a slide rail.
For example, systems for determining the position of a carriage in relation to a slide rail of the aforementioned kind are used in combination with guide systems, e.g., linear guides, which encompass a first body and a second body guided on the first body that can move relative to the first body, and here have the job of making it possible to determine the position of the second body relative to the first body. To this end, for example, a measuring scale of the respective device for determining a position can be fixed in place relative to the first body, for example, and a respective scanner can be fixed in place relative to the second body.
For example, linear position measuring systems for determining an absolute position are known in the art, which encompass the measuring scale marked with measuring points, and a scanner that can move relative to the measuring scale for scanning the respective measuring points. For example, these measuring points consist of one or more acquirable markings to identify a position. The markings can be acquired optically or magnetically, for example.
In the case of optical scanning, the scanner encompasses a sensor for acquiring an image of the measuring points and providing signals making it possible to determine the position of the scanner relative to the measuring scale. In the case of magnetic scanning, the scanner encompasses a magnetic field sensor for acquiring a magnetic field progression of individual permanent magnets, which in this case make up the measuring points of the measuring scale.
Depending on the respective measuring scale (optical/magnetic), these types of systems can be used, for example, to measure a relative change in the position of the scanner in relation to an initial position.
To reach a point where these types of systems become able to measure relative changes in position of the scanner in relation to the measuring scale, for example, the respective measuring scale can be designed as an incremental scale, and consequently acquire a sequence of several identical, periodically arranged markings spaced apart at equal distances along a prescribed line or measuring scale. For example, to enable the optical scanning of such an incremental measuring scale, the scanner can project an optical image of the respective markings onto a sensor in the form of a photoelectric detector. To measure relative changes in position of the scanner in relation to the measuring scale, the scanner is moved along the track of markings. Moving the scanner here causes a signal to change periodically, for example providing information about how many markings the scanner passes by within a predetermined time.
In sum, the respective change in the relative position of the scanner can be determined by scanning the measuring points or incremental markings of an incremental measuring scale. So-called incremental position encoders having a comparatively simple design and a high resolution are used for this purpose.
Scanners in known linear position measuring systems must be continuously supplied with external electrical power to prevent the loss of stored information about the respective position of the scanner. Even just a brief power interruption without any power buffer would cause information about the respective position of the scanner to be lost. As a consequence, even after the external power supply is resumed, the scanner would output erroneous information about the position, or none at all. In order to return to flawless operation from this state, the carriage would have to undergo a time-intensive recalibration in relation to the slide rail.
As known, scanners are equipped with a mobile (internal) power source, for example a battery or accumulator, which at least intermittently supplies power to a data storage device of the scanner to store information about position given an interruption in external power supply. The interruption in external power supply can here be bridged, thereby ensuring the flawless operation of the external power supply for the duration of the interruption and even beyond.
The maximum time for which the power can be bridged depends on the capacity of the mobile power source. As the capacity of the mobile power source grows (holding power consumption constant), so does the maximum time for which the interruption in external power supply can be bridged, too. The disadvantage here is that a rising capacity for the mobile power source is accompanied by an increase in its costs. In addition, the use of mobile power sources having an elevated capacity is also associated with an increased weight and/or increased space requirement. In particular the two last-mentioned factors have an overall negative impact on the operation of the linear position measuring system. For example, the increased weight of the mobile power source diminishes the overall acceleration of the carriage.